Concrete, which is made from a hydraulic cement binder and fine and course aggregates, is known to be a fairly brittle material. If its maximum tensile strength is exceeded, then crack initiation and propagation will occur. The concepts of "flexural strength" and "fracture toughness" are useful for understanding crack behavior in concrete.
Flexural strength is related to the critical stress intensity factor, e.g., the ability of a structure to resist crack initiation. Since it is proportional to the maximum sustainable load, flexural strength is measured as the minimum load or stress required to initiate or start a crack under flexural loading.
Fracture toughness is related on the other hand to the specific "fracture energy" of a concrete, e.g., the ability to resist the propagation or widening of an opened crack. This toughness property is proportional to the energy required to propagate or widen a crack. This property can be determined by simultaneously measuring the load, which is required to deform or "deflect" a fiber-containing concrete (FRC) sample at an opened crack, and the amount of deflection. Toughness is therefore determined by dividing the area under a load deflection curve generated from plotting the load against deflection of the specimen by its cross-sectional area.
The "ductility" of a material is closely related to the characteristic length l.sub.ch. which is directly proportional to the ratio of the fracture energy, G.sub.F, and the stored elastic energy at the maximum load, G.sub.Ic (l.sub.ch .varies.G.sub.F /G.sub.Ic).
The fracture toughness or energy of non-reinforced concrete is very low, somewhere in the range of 50 to 200 N/m. This low fracture toughness is the main reason for the high brittleness of concrete in tension and compression. Once the breaking point of non-reinforced concrete (under tension) is reached, the concrete cracks and fails (crumbles). However, it is known to use reinforcing fibers in concrete to increase the amount of energy required to bring the concrete to a state of complete separation of its fracture surfaces. Various fibers made of steel, polyolefin, carbon, nylon, aramid, and glass have been suggested for such use.
In an article entitled "Flexural Characteristics of Steel Fibre and Polyethylene Fiber Hybrid-Reinforced Concrete," Kobayashi and Cho described a fiber-reinforced concrete made by dispersing discontinuous steel and polyethylene fibers in a randomly oriented state into the concrete to provide it with both strength and toughness. K Kobayashi and R. Cho, Composites, Vol. 13 (Butterworth & Co. Ltd. 1982), pp. 164-168.
Kobayashi and Cho used one (1) percent by volume of steel fibers made by shearing cold-rolled steel, the dimensions being 0.35 mm.times.0.7 mm.times.30 mm, and one-three percent by volume of polyethylene fibers having a length of 40 mm and a (circular) diameter of 0.9 mm. The steel fibers provided flexural strength by resisting crack initiation, and the polyethylene fibers provided fracture toughness by providing pull-out resistance and viscoelastic ability. This hybrid steel/polyolefin system overcame the singular deficiencies of either steel or polyolefin fibers used alone. In other words, steel fibers increased first-crack strength which the polyethylene fibers did not do when used alone; while the polyethylene fibers increased strength after crack formation which the steel fibers did not do when used alone. However, Kobayashi and Cho taught that their steel fibers should be used at one percent (1%) volume, above which there was extreme loss of fluidity in the concrete.
In World Patent Application WO 98/27022, J. Seewald disclosed a high strength concrete having enhanced ductility using 30-200 kg/m3 of inorganic (e.g., steel) fibers (approximately 0.4-2.6 percent volume) along with a smaller amount of organic fibers having a low elasticity modulus. Although Seewald taught using preferably seven times as much steel fibers as polypropylene fibers, it is not clear how he resolved any fluidity problems that would certainly have been the concern of Kobayashi and Cho, as just noted above.